The biggest and strangest batteries

The last one (ice storage air conditioning) is going to be a huge one. It doesn't depend on cycling some useless process (like moving water/rocks/air/electrons in and out of some potential gradient) -- rather, it uses a process that already needs to happen (air conditioning and chilling) and runs it at a higher efficiency and a lower price, by using cheaper night electricity and colder outside temperatures.

It also scales neatly up and down with the space being cooled; there's no inefficiencies at small scales. It's possible to add such a system (with an appropriate phase-change material) to most any air-conditioning installation -- as long as there's a varying load (if it runs 24/7 with the same load, ice storage won't do anything useful) and the system isn't weight-sensitive (i wouldn't put this aboard an aeroplane, say), there's a potential.

Another nice gravity based one: http://www.heindl-energy.com/gravity-storage/idea-function.h...

A piston of rock carved out of the land. For now just a concept though.